A known class of lamps excites a gas causing it to break down to a light emitting plasma. The well-known fluorescent lamp falls into this class. With a fluorescent lamp, a direct current (DC) is applied to a pair of electrodes in a glass tube filled with a mixture of argon and mercury. Another type of lamp falling within this class is the electrodeless lamp. As discussed in, for example, U.S. Pat. No. 5,013,959 to Kogelschatz issued May 7, 1991, an electrodeless lamp may comprise a pair of concentric dielectric tubes. An electrode is disposed within the inner tube and another surrounds the outer tube; the gas is contained in the annulus between the tubes. An alternating current (AC) is applied to the electrodes to create an electric field in the gas thereby exciting the gas. This type of lamp is known as "electrodeless" because the electrodes do not contact the gas.
Electrodeless lamps may be filled with gas which emits in the UV spectrum. In such case, the lamp will be arranged to direct light to a treatment chamber so as to treat material in the chamber with UV light.
It is desirable to maximize the light generated by the lamp, however, the outer electrode may block some light from leaving the lamp. It is also desirable to maximize lamp life, however, the high energy plasma ions may degrade the dielectric tubes.
This invention seeks to overcome drawbacks of known electrodeless lamps.